Electromagnetic ultrasound converter

ABSTRACT

Disclosed is an electromagnetic ultrasonic transducer for coupling-media-free generation and/or reception of ultrasonic waves in the form of linearly polarized transverse waves in a, respectively from a workpiece, having at least one unit which converts the ultrasonic waves inside the workpiece and which is provided with a coil arrangement for generating, respectively detecting a high-frequency magnetic field as well as with a premagnetizing unit for generating a quasi-static magnetic field which superimposes upon the high-frequency magnetic field in the workpiece, with the coil arrangement being disposed in a torus-shaped manner on at least one partially toroidal or U-shaped magnetic core, which each has two front ends which can be turned to face the workpiece. The invention is distinguished in that the front ends of the magnetic core which can be turned to face the workpiece are connected directly or indirectly to a magnetic flux piece which has a surface which faces the workpiece and which connects the front ends with each other.

BACKGROUND OF THE INVENTION

The present invention relates to an electromagnetic ultrasonictransducer for coupling-media-free generation and/or reception ofultrasonic waves in the form of linearly polarized transverse waves ina, respectively from, a workpiece having at least one unit that convertsthe ultrasonic waves inside the workpiece. The unit has a coilarrangement for generating, respectively detecting, a high-frequencymagnetic field and a premagnetizing unit for generating a quasi-staticmagnetic field which superimposes the HF magnetic field in the workpiece, with the coil arrangement being disposed in a torus-shaped manneron at least one partially toroidal or U-shaped designed magnetic corewhich has two front ends which can each be turned to face the workpiece.

Such type ultrasonic probes permit generating and receiving linearlypolarized transverse waves, which preferably are irradiated under theprobe perpendicularly into the workpiece, respectively are received fromthis direction and oscillate preferably perpendicular to theirpropagation direction. Technical fields of application of such typeultrasonic probes are, for example, nondestructive examination ofelectrically conductive workpieces for material flaws, such as forexample cracks, in particular crack-like flaws oriented parallel to thepolarization direction of the ultrasonic waves and perpendicular to thepropagation direction, as well as other processes based on ultrasonicvelocity and polarization, such as for example measuring voltage or, inparticular, measuring thickness.

PRIOR ART

The coupling-media-free electromagnetic probes known from the state ofthe art convert electromagnetic field energies in the elastic energy ofan ultrasonic wave and inversely. The conversion mechanism is based onthe interaction between the electromagnetic field and an electricallyconducting material that moreover a static magnetic field or aquasi-static magnetic field applied from the outside passes through. Theterm “quasi-static” magnetic field comprises, in addition to the actualstatic magnetic field, which for example can be generated by means ofpermanent magnets, also low-frequency magnetic fields, whose alternatingfrequency is much lower than the high frequency with which the coilarrangement is operated to generate high-frequency fields.

In order to excite ultrasonic waves inside an electrically conductingworkpiece, at least one part of the high-frequency magnetic field, whosefrequency range lies within the ultrasonic frequency range, generated bythe high-frequency coil arrangement is coupled into the workpiece, thusinducing eddy currents at skin depth which if superimposed by the“quasi-static” magnetic field generate ultrasonic waves due to theLorentz forces or magnetostrictions occurring inside the workpiece.

Detection of ultrasonic waves occurring inside the workpiece occursinversely by detection of the electric voltage induced inside the coilarrangement resulting from high-frequency fields which for their partare generated by the motions of electric charges, due to the ultrasonicwaves, in the workpiece inside the “quasi-static” magnetic field.

All prior art electromagnetic ultrasonic transducers are based on thecommon goal of development to optimize measuring sensitivity and,related thereto, the signal amplitudes in both the transmission signalas well as in the reception signal that can be generated with the coilarrangements. The aim, on the one hand is to design the couplingmechanism with which the generated and to-be-detected high-frequencyfields are coupled in and out between the ultrasonic transducer and theworkpiece as loss-free as possible and, on the other hand, to select thefield strength of the quasi-static magnetic field as large as possible,which is decisive for generating and detecting ultrasonic waves.

DE 42 23 470 C2 describes a generic electromagnetic probe for verticalacoustic irradiation of linearly polarized transverse waves, in whichthe high-frequency magnetic fields are coupled in and out in a mostefficient manner between the probe and the workpiece without, as is thecase with many other probes, placing the transmission and receptioncoils, usually designed as high-frequency air coils, directly on thesurface of the workpiece. But rather the electromagnetic probe of FIG. 2described in this printed publication is provided with a half-opentoroidal tape core 1, made commercially of amorphous tape material,around which a transmission coil 41 and a reception coil 42,respectively, are wound. The front ends 2 of the half-open toroidal tapecore 1 act as coupling areas for the high-frequency magnetic fields andcan be placed in a suited manner on the surface of the to-be-examinedworkpiece 7. The high-frequency magnetic fields generated by thehigh-frequency transmission coil arrangement 41 reach via the front ends2 of the toroidal tape core 1 into the workpiece 7 and are able toinduce close-to-the surface eddy currents 8 at skin depth inside theworkpiece 7.

The quasi-static magnetic field oriented perpendicular to the surface ofthe workpiece 7 required for sound conversion is generated by means oftwo permanent magnets 6 of the same name and conveyed to the materialsurface of the workpiece 7. The premagnetizing unit required forproducing the “quasi-static” magnetic field that is orientedperpendicular to the surface of the workpiece is located inside the openpart of the toroidal tape core 1. With this arrangement, ultrasonicwaves with a propagation direction A perpendicular to the surface of theworkpiece and an oscillation plane S perpendicular thereto developinside the workpiece.

DE 41 30 935 A1 describes a probe device comparable to this arrangement.However, in this probe device the transmission and reception coilarrangement lies directly on the surface of the to-be-examinedworkpiece, which harbors the danger of coil wear.

DE 195 43 482 A1 describes a device for testing ferromagnetic materials,preferably in the form of pipe lines. However, this device has acomponents setup that differs from the state of the art described indetail in the preceding and on which the following is based.

SUMMARY OF THE INVENTION

Based on the aforementioned state of the art, the object of the presentinvention is to further develop a generic electromagnetic ultrasonictransducer in such a manner that the efficiency with which theultrasonic waves are generated and detection sensitivity are decisivelyimproved compared to the prior art ultrasonic transducers. Inparticular, the object is to dispose the coil arrangement at a distancefrom the surface of the workpiece in order to be able to rule outmechanical impairment of the coil arrangement. Moreover, the furtherdeveloped ultrasonic transducer is to permit generating horizontallypolarized ultrasonic waves.

A key element of the present invention is to further develop anelectromagnetic ultrasonic transducer for coupling-media-free generationand/or reception of ultrasonic waves in the form of linearly polarizedtransverse waves in, respectively from, a workpiece, having at least oneunit which converts the ultrasonic waves inside the workpiece and whichis provided with a coil arrangement for generating, respectivelydetecting, a high-frequency magnetic field as well as a premagnetizingunit for generating a quasi-static magnetic field which superimposesupon the high-frequency magnetic field in the workpiece, with the coilarrangement being disposed in a torus-shaped manner on at least onepartially toroidal or U-shaped magnetic core, which has two front endsthat can be turned to face the workpiece, in such a manner that thefront ends of the magnetic core that can be turned to face the workpieceare connected directly or indirectly to a magnetic flux guide piecewhich has a surface facing the workpiece and which connects the frontends with one another.

Providing such a type flux guide piece connecting the front ends of amagnet core, preferably designed as a toroidal tape core, with eachother permits, in particular, coupling-in the high-frequency magneticfields generated by the coil arrangement most efficiently into theworkpiece in order to be able, in this manner, to generate verydistinctive eddy currents at skin depth. For this purpose, the fluxguide piece has a surface which is preferably designed conform with thesurface of the workpiece thereby permitting, preferablycontour-matching, contacting of the flux guide piece and the workpiece.In a preferred embodiment, the flux guide piece is formed as arectangular rod with a plane surface facing the workpiece. The planesurface can be placed flush on an equally plane formed workpiece surfacewithout any coupling media. The surface of the flux guide piece facingthe workpiece can, of course, be produced, depending on the curvaturebehavior of the to-be-examined workpieces, in a surface mold matchingthe contour of the workpiece. If the electromagnetic ultrasonictransducer is to be, for example, utilized preferably for examiningcylindrical workpiece surfaces, the flux guide piece connecting thefront ends of the toroidal tape cores is formed corresponding to thenature of the contour.

In addition to optimized coupling-in of the high frequency magneticfield into the workpiece surface by means of the flux guide piece, theflux guide piece is also able to couple-in the quasi-static magneticfield almost without losses into the workpiece. For this purpose, in atypical preferred embodiment the premagnetizing unit for generating thequasi-static magnetic field has the form of a permanent magnet, which isdisposed directly on the flux guide piece between the front ends of thetoroidal tape core, which projects beyond the permanent magnet. In thiscase, the flux guide piece acts as a type concentrator for thequasi-static, respectively permanent magnetic field.

In order to prevent eddy currents from developing inside the flux guidepiece, it is advantageous to make the flux guide piece out of anelectrically nonconductive carrier material into which matrix-like softmagnetic particles are introduced. Alternatively, a stack-shapedarrangement of soft magnetic transformer metal sheets can alsoeffectively prevent eddy currents from developing inside the flux guidepiece.

In addition to the aforedescribed invented design of an electromagneticultrasonic transducer whose partially toroidal or U-shaped magnetic coreprojects beyond a single magnetic flux guide piece, with the front endsof the magnetic core being closely connected to the single flux guidepiece, a second, alternative invented preferred embodiment of anelectromagnetic transducer is provided with at least two flux guidepieces disposed side by side in parallel which are connected to eachother bridge-like by at least two partially toroidal or U-shapedmagnetic cores via their respective front ends. The magnetic cores aresituated at a distance from each other at the opposite end regions ofthe rod-shaped flux guide pieces. This manner of construction permitsplacing the premagnetizing unit, preferably in the form of a permanentmagnet arrangement, between the two magnetic cores in longitudinaldirection of the two flux guide pieces without the magnetic cores,preferably designed as toroidal tape cores, spanning them as is the casein the aforedescribed preferred embodiment. As a result, thepossibilities in scaling the dimensions of the permanent magnetarrangement are practically unlimited, permitting enlarging the magneticfield strength accordingly.

In addition to the simplest preferred embodiment of the magnetic core inthe form of a toroidal tape core wound with at least one coilarrangement, also feasible are magnetic cores with an M-shaped magneticcross section each having three free-ending front ends. Compared to thepreceding electromagnetic ultrasonic transducer arrangement, with suchtype magnetic cores three parallel adjacent magnetic flux guide piecescan be connected in a bridge-like manner.

As described in detail further on herein with reference to the followingpreferred embodiments, linearly polarized transverse waves can begenerated inside the workpiece by suited combination of a multiplicityof the aforedescribed ultrasonic transducers and triggering of the coilarrangements placed on the magnetic cores in a toroidal manner. Inparticular, corresponding multiple arrangements permit generatingvertically or horizontally linearly polarized transverse waves.

For this purpose the electromagnetic ultrasonic transducers according tothe present invention described in the beginning can be placed side byside in multiple arrangement in order to obtain, on the one hand, alarge as possible transmitting and receiving aperture and, on the otherhand, to obtain, by means of phase-controlled high-frequency excitationof the individual coil arrangements, a selectively settable irradiationcharacteristic for the ultrasonic waves that can be coupled into theworkpiece. As will be described further on herein, such typearrangements are suited for a phased array arrangement for generatinghorizontally polarized transverse waves (shear horizontal waves) whosepropagation direction can be selectively set, which includes withreference to the normals of the workpiece surface a variable anglebetween 0° and 90°.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is made more apparent in the following withreference to the accompanying drawings by way of example without theintention of limiting the scope or spirit of the inventive idea.

FIG. 1 shows an electromagnetic ultrasonic transducer having a singleflux guide piece,

FIG. 2 shows a state-of-the-art electromagnetic ultrasonic transducer,

FIG. 3 shows an arrangement of a multiplicity of single electromagneticultrasonic transducers according to the embodiment of FIG. 1,

FIG. 4 shows a schematic representation of an ultrasonic wave fieldgenerated inside a workpiece by the arrangement according to FIG. 3,

FIG. 5 shows an electromagnetic ultrasonic transducer having two fluxguide pieces which are spanned bridge-like by two semi-toroidal magneticcores,

FIG. 6 shows a multiplicity of electromagnetic ultrasonic transducersaccording to FIG. 5 and

FIG. 7 shows a cross section of M-shaped magnetic cores which each spanbridge-like three magnetic flux guide pieces.

WAYS TO CARRY OUT THE INVENTION, COMMERCIAL APPLICABILITY

FIG. 1 shows the simplest preferred embodiment of an electromagneticultrasonic transducer designed according to the present invention, inwhich the partially toroidally designed magnetic core is designed as ahalved toroidal tape core 1 whose two front ends 2 are directlyconnected to a rod-shaped flux guide piece 3 designed with a rectangularcross section. Wound about the halved toroidal tape core 1 in atorus-shaped manner is a coil arrangement 4, which has two connectingcontacts 5. Provided directly on the surface 31 of the flux guide piece3 is a premagnetizing unit 6, which, in the preferred embodiment, isdesigned as a permanent magnet and has a north pole as indicated. Thetoroidal tape core 1 projects completely over the permanent magnet 6.Also only indicated is the workpiece 7 to be examined with the aid ofthe electromagnetic ultrasonic transducer arrangement and on whoseworkpiece surface 71, the flux guide piece 3 with its surface 32 facingthe workpiece 7 can be placed, preferably matching its contour. In thismanner, the magnetic field generated by means of the permanent magnet 6enters the workpiece 7 perpendicularly through the flux guide piece 3via the workpiece surface 71. Alternatively to the embodiment of thepremagnetizing unit in the form of a permanent magnet 6, as depicted inFIG. 1, it is feasible to design the premagnetizing unit in the form ofan electromagnet whose field lines enter the workpiece perpendicular tothe workpiece surface in the same manner as in the arrangement depictedin FIG. 1. However, it is also possible to position an electromagnet insuch a manner that the magnetic field generated by the electromagnetenters the workpiece parallel to the workpiece surface. The effectsrelated to such a type magnetic field alignment is described in thefurther on herein.

The electromagnetic ultrasonic transducer shown in FIG. 1 should beviewed as an electromagnetic line transducer which can be operated bothas an ultrasonic transmitter and an ultrasonic receiver. Whentransmitting, the connecting contacts 5 are connected to ahigh-frequency generator, when receiving however the connecting contacts5 are connected to a corresponding amplifier and a downstream evaluationunit. Of course, two separate coil arrangements, of which one acts as atransmission coil and the other as a reception coil, can also beprovided along a single halved toroidal tape core.

Due to the large area contact between the upper side 32 of the fluxguide piece 3 and the workpiece surface 71 of the workpiece 7, thehigh-frequency magnetic fields conveyed in the longitudinal direction ofthe flux guide piece 3 couple into the workpiece 7 along the entirelongitudinal extension of the flux guide piece 3 and generate intensiveeddy currents at skin depth. These eddy currents, for their part,interact with the quasi-static magnetic field passing through theworkpiece surface 71 and generate, due to the developing Lorentz forcesand magnetostrictions, ultrasonic waves with a frequency correspondingto the alternating frequency of the high-frequency magnetic fields. Dueto the close contact between the flux guide piece 3 and the workpiecesurface 71, a higher magnetic flux is generated inside the workpiece 7than is the case with the hitherto known electromagnetic ultrasonictransducers, for example as depicted in FIG. 2. In this manner, theeffectivity and the generation of ultrasonic waves as well as thesensitivity in the reception case can be increased considerably.

The preferred embodiment of an electromagnetic ultrasonic transducershown in FIG. 1, in which the quasi-static magnetic field passesperpendicularly through the workpiece surface 71, permits generatinglinearly polarized transverse waves whose propagation direction isoriented perpendicular to the surface of the workpiece and has anoscillation plane oriented perpendicular to the propagation direction.

Selective excitation of so-called horizontally polarized transversewaves (shear horizontal waves) requires, in an as such known manner, apremagnetizing unit, usually in the form of an arrangement of permanentmagnets with alternating polarity whose alternating magnetic fieldssuperimpose a high-frequency magnetic field inside the workpiece. FIG. 3shows an arrangement designed according to the present invention forgenerating horizontally polarized transverse waves, which, in thedepicted preferred embodiment, is provided with five parallelelectromagnetic line transducers arranged adjacent to each otheraccording to the example shown in FIG. 1. It is assumed that the coilarrangements 4, as indicated in FIG. 3, on each of the singly depictedultrasonic transducers are designed for generating as well as forreceiving ultrasonic waves. If the electrical connections 5 of theindividual coil arrangements 4 for transmitting and receiving areconnected at separate electronic channels of a high-frequency generator,respectively of a corresponding amplifier, and if the individualelectronic channels are operated time-delayed in their triggering phase,a phased array arrangement which is able to generate and detecthorizontally polarized transverse waves inside the workpiece—like agroup radiator—can be realized with the arrangement depicted in FIG. 3.Adjacently disposed ultrasonic transducers operated with a magnetic fluxdirected in the opposite direction in the flux guide piece, therebygenerating alternating developing eddy current directions under theadjacent flux guide pieces, which lead to Lorentz forces directed inopposite directions and the related magnetostriction directions and inthis manner generate shear forces to produce horizontally polarizedtransverse waves inside the workpiece.

Suited selection of the phase-dependent triggering of the individualultrasonic transducers disposed side by side in a row permitsselectively setting the direction characteristic of the developinghorizontally polarized transverse waves in such a manner that the mainpropagation direction of the main lobe of the horizontally polarizedtransverse waves form an angle α, selectable from 0° to 90° as desired,with the surface normals of the surface of the workpiece. FIG. 4 showsan illustrative sketch of generating horizontally polarized transversewaves with the aid of the ultrasonic wave arrangement depicted in FIG.3. It is assumed that four ultrasonic transducers S₁-S₄ are disposedside by side at a distance D from each other on the workpiece surface71. A current pulse is applied to the individual ultrasonic transducersS₁-S₄ at an interval of Δt in the aforedescribed manner. Due to thephase-delayed application of current to the ultrasonic transducersS₁-S₄, horizontally polarized transverse waves develop inside theworkpiece. These transverse waves have a main propagation direction thatforms with the normals of the surface of the workpiece an angle α, forwhichα=sin(c _(t) ·Δt/D)applies.

In the above equation c_(t) stands for the propagation velocity of thehorizontally polarized transverse wave in the workpiece,

showing in this manner that if all four ultrasonic transducers aretriggered phase-synchronously, i.e. Δt=0, α equals zero so that thehorizontally polarized transverse waves are irradiated into theworkpiece perpendicular to the surface of the workpiece. If theindividual ultrasonic transducers are operated with a phase-delay of Δt,during which an ultrasonic wave, for example, reaches from an ultrasonictransducer S1 to the transducer S2, that is covers the distance D, themain lobe of the horizontally polarized transverse waves developinginside the workpiece forms an angle α of 90° with the normals of thesurface of the workpiece. Depending on the choice of Δt, the main lobecan be swung as desired between 0° and 90° inside the workpiece.

The preceding description, which relates to a transmission operation,can be transferred inversely also to the reception of shear horizontalwaves from a workpiece.

Another embodiment of an electromagnetic ultrasonic transducer designedaccording to the present invention is shown in FIG. 5. The transducer isprovided with two magnetic flux guide pieces 3, 3′ disposed in parallelat a distance from each other. The upper sides 32, 32′ of the twomagnetic flux guide pieces 3, 3′ are connected to the front ends of thetwo semi-circular-shaped toroidal tape cores 1, 1′. The two magneticflux guide pieces 3, 3′ are thus connected to each other in abridge-like manner by the toroidal tape cores 1,1′. Moreover, thepreferred embodiment shown in FIG. 5 has two counter pole permanentmagnets 6, 6′ resting on the respective magnetic flux guide pieces 3,3′. Triggering the coil arrangements of the individual toroidal tapecores 1,1′ occurs in such a manner that dynamic magnetic fields directedin opposite directions are generated in the magnetic flux guide pieces3, 3′, leading to eddy currents in the workpiece, which are orientedperpendicular to the longitudinal extension of the magnetic flux guidepieces 3, 3′ as well as in opposite directions. Due to the oppositepoled permanent magnets 6, 6′, shear forces directed in the samedirection develop at skin depth inside the workpieces under the fluxguide pieces, thereby creating linearly polarized transverse waves withultrasonic waves propagating perpendicular to the surface of theworkpiece. Therefore, the arrangement shown in FIG. 5 can be viewed as anormal probe for generating and detecting linearly polarized transversewaves having a large aperture, comparable to the arrangement which hasonly a single electromagnetic line transducer according to the type ofsetup of the preferred embodiment shown in FIG. 1.

FIG. 6 shows an arrangement of three normal probes disposed side by sideaccording to the basic setup of the ultrasonic transducer depicted inFIG. 5. The three normal probes N disposed side by side are switchedwith their respective high-frequency coils 4 in such a manner that inthe adjacent flux guide pieces, the direction of the dynamic magneticfields are oriented in opposite directions, i.e. there is a phasedifference of 180° between the directly adjacent high-frequency magneticfields. Due to the in this manner oriented magnetic flux of the dynamicmagnetic fields directed in opposite directions, in transmission eddycurrents which are oriented perpendicular to the direction of thedynamic magnetic fields are coupled into the surface of the workpiece.If a uniform static magnetic field is superimposed, the eddy currents jgenerate Lorentz forces F_(I) directly under the respective flux guidepieces 3. The Lorentz forces F_(I) under the adjacent flux guide piecesare directed in opposite directions and therefore produce shear forcesinside the workpiece thereby generating shear horizontal transversewaves. The coil wavelength which corresponds to the half oscillationwavelength of the SH wave, is determined by the distance between thedirectly adjacent flux guide pieces. The irradiation direction of theshear horizontal waves is oriented perpendicular to the individual fluxguide pieces 3, indicated by the arrows A directed in oppositedirections according to FIG. 6.

The arrangement shown in FIG. 6 comprises, in particular, also aselective use of a large-area unipolar magnet 6 which is decisively ableto suppress the disturbing Barkhausen noise in the region of theindividual transducer elements.

FIG. 7 shows an arrangement very similar to the arrangement of FIG. 6for generating shear horizontal waves. Contrary to FIG. 6, in FIG. 7 thetoroidal tape core segments 1 are designed in the form of m-shaped coilcores, with two m-shaped coil cores each being provided with three fluxguide pieces 3, 3′, 3″. The coil arrangements provided around the coilcores 1 are switched in such a manner that high-frequency magneticfields directed in opposite directions develop in the longitudinaldirection of the flux guide pieces located adjacent to each other inparallel. Superimposition of the high-frequency magnetic fields by astatic magnetic field generated by a unipolar permanent magnet 6, whichcompletely covers the three normal probes N, and oriented perpendicularto the surface of the workpiece over the high-frequency magnetic fieldsleads to eddy currents directed in opposite directions inside theworkpiece under the directly adjacent flux guide pieces, thereby evokingLorentz forces which are also directed in opposite directions and, fortheir part, are responsible for the shear forces required to generatehorizontally polarized transverse waves.

List of References

1 toroidal tape core

2 front end

3 flux guide piece

31,32 surfaces of the flux guide piece

4 coil arrangement

41 transmission coil

42 reception coil

5 electrical contacts

6 permanent magnet

7 workpiece

8 eddy current

1-18. (canceled)
 19. An electromagnetic ultrasonic transducer forcoupling-media-free generation and/or reception of ultrasonic waves inthe form of linearly polarized transverse waves in a, respectively froma workpiece, having at least one unit which converts the ultrasonicwaves inside the workpiece and which is provided with a coil arrangementfor generating, respectively detecting a high-frequency magnetic fieldas well as with a premagnetizing unit for generating a quasi-staticmagnetic field which superimposes the high-frequency magnetic field inthe workpiece, with the coil arrangement being disposed in atorus-shaped manner on at least one partially toroidal or U-shapedmagnetic core, which each has two front ends which can be turned to facethe workpiece, wherein the front ends of the magnetic core which can beturned to face the workpiece are connected directly or indirectly to amagnetic flux piece which has a surface which faces the workpiece andwhich connects the front ends with each other.
 20. An electromagneticultrasonic transducer for coupling-media-free generation and/orreception of ultrasonic waves in the form of linearly polarizedtransverse waves in, respectively from a workpiece, having at least oneunit which converts the ultrasonic waves inside the workpiece and whichis provided with a coil arrangement for generating, respectivelydetecting a high-frequency magnetic field as well as with apremagnetizing unit for generating a quasi-static magnetic field whichsuperimposes the high-frequency magnetic field in the workpiece, withthe coil arrangement being disposed in a torus-shaped manner on at leastone partially toroidal or U-shaped magnetic core, which each has twofront ends which can be turned to face the workpiece, wherein at leasttwo magnetic cores and at least two magnetic flux guide pieces areprovided, one front end of a magnetic core is connected directly orindirectly to one of the at least two magnetic flux guide pieces and theother front end of a magnetic core is connected directly or indirectlyto the other of the at least two magnetic flux guide pieces, one frontend of the other magnetic core is connected directly or indirectly toone of the at least two magnetic flux guide pieces and the other frontend of the other magnetic core is connected directly or indirectly tothe other of the at least two magnetic flux guide pieces each located ata distance from the first magnetic core, and the magnetic flux guidepieces each have a surface facing the workpiece or an electromagneticarrangement.
 21. The electromagnetic ultrasonic transducer according toclaim 20, wherein the at least two magnetic cores are each designedU-shaped and each have two longitudinal limbs connected via a connectionpart, the front ends being provided at the ends of the two longitudinallimbs; between the longitudinal limbs at least one further longitudinalelement is provided which is connected on one side to the connectionpart at the end of which a further front end is provided and, at leastone further magnetic flux guide piece connects the front ends of thelongitudinal elements of both magnetic cores with each other.
 22. Theelectromagnetic ultrasonic transducer according to claim 19, wherein togenerate ultrasonic waves the coil arrangement is connected to ahigh-frequency generator to generate high-frequency magnetic fields. 23.The electromagnetic ultrasonic transducer according to claim 20, whereinto generate ultrasonic waves the coil arrangement is connected to ahigh-frequency generator to generate high-frequency magnetic fields. 24.The electromagnetic ultrasonic transducer according to claim 19, whereinto detect ultrasonic waves the coil arrangement is connected to anamplifier unit and/or to an evaluation unit.
 25. The electromagneticultrasonic transducer according to claim 20, wherein to detectultrasonic waves the coil arrangement is connected to an amplifier unitand/or to an evaluation unit.
 26. The electromagnetic ultrasonictransducer according to claim 19, wherein the coil arrangement providesto separate coils, a transmission coil arrangement to generate ahigh-frequency magnetic field which is connected to a high-frequencygenerator and a reception coil arrangement for detecting ahigh-frequency magnetic field which is connected to an amplifier unitand/or to an evaluation unit.
 27. The electromagnetic ultrasonictransducer according to claim 20, wherein the coil arrangement providestwo separate coils, a transmission coil arrangement to generate ahigh-frequency magnetic field which is connected to a high-frequencymagnetic generator and a reception coil arrangement for detecting ahigh-frequency magnetic field which is connected to an amplifier unitand/or to an evaluation unit.
 28. The electromagnetic ultrasonictransducer according to claim 19, wherein the magnetic flux guide pieceis designed rod-shaped and contains soft magnetic material.
 29. Theelectromagnetic ultrasonic transducer according to claim 20, wherein themagnetic flux guide piece is designed rod-shaped and contains softmagnetic material.
 30. The electromagnetic ultrasonic transduceraccording to claim 19, wherein the magnetic flux guide piece comprises astack-shaped arrangement of soft magnetic board elements or is made ofan electrically nonconductive material containing soft magneticparticles distributed in the form of a matrix.
 31. The electromagneticultrasonic transducer according to claim 19, wherein the magnetic fluxguide piece comprises a stack-shaped arrangement of soft magnetic boardelements or is made of an electrically nonconductive material containingsoft magnetic particles distributed in the form of a matrix.
 32. Theelectromagnetic ultrasonic transducer according to claim 19, wherein thefront ends of the magnetic core are firmly fused with the magnetic fluxguide piece.
 33. The electromagnetic ultrasonic transducer according toclaim 20, wherein the front ends of the magnetic core are firmly fusedwith the magnetic flux guide piece.
 34. The electromagnetic ultrasonictransducer according to claim 19, wherein the permagnetizing unit isprovided directly or indirectly on an upper side of the at least onemagnetic flux guide piece facing away from the workpiece.
 35. Theelectromagnetic ultrasonic transducer according to claim 20, wherein thepermagnetizing unit is provided directly or indirectly on an upper sideof the at least one magnetic flux guide piece facing away from theworkpiece.
 36. The electromagnetic ultrasonic transducer according toclaim 19, wherein the premagnetizing unit is a permanent magnet or anelectromagnetic arrangement.
 37. The electromagnetic ultrasonictransducer according to claim 20, wherein the premagnetizing unit is apermanent magnet or an electromagnetic arrangement.
 38. Theelectromagnetic ultrasonic transducer according to claim 19, wherein thepremagnetizing unit is disposed in such a manner that the quasi-staticmagnetic field can be introduced into the workpiece perpendicular to thesurface of the workpiece.
 39. The electromagnetic ultrasonic transduceraccording to claim 20, wherein the premagnetizing unit is disposed insuch a manner that the quasi-static magnetic field can be introducedinto the workpiece perpendicular to the surface of the workpiece. 40.The electromagnetic ultrasonic transducer according to claim 19, whereinthe premagnetizing unit is an electromagnetic arrangement by means ofwhich a quasi-static magnetic field can be introduced into the workpiecehorizontally to the surface of the workpiece.
 41. The electromagneticultrasonic transducer according to claim 20, wherein the premagnetizingunit is an electromagnetic arrangement by means of which a quasi-staticmagnetic field can be introduced into the workpiece horizontally to thesurface of the workpiece.
 42. An arrangement for coupling-media-freegenerating and/or for receiving ultrasonic waves in the form of linearlypolarized transverse wave in a, respectively from a workpiece, whereinat least two electromagnetic ultrasonic transducers according to claim19 are disposed in parallel at a distance to each other in such a mannerthat the longitudinal directions of the respective magnetic flux guidepieces of the individual ultrasonic transducers are aligned in parallel.43. The arrangement according to claim 42, wherein the premagnetizingunits of the individual ultrasonic transducers are of the same name, ora single premagnetizing unit extends over all the magnetic flux guidepieces of the side-by-side disposed ultrasonic transducers.
 44. Use ofthe arrangement according to claim 42 for generating and/or detectinghorizontally polarized transverse waves, wherein the coil arrangementsof the at least two electromagnetic ultrasonic transducers are operatedby means of a phase-array triggering.
 45. Use according to claim 44,wherein the coil arrangements of all of the electromagnetic ultrasonictransducers are triggered consecutively with a time-delayed phasetrigger signal in such a manner that when ultrasonic waves are generatedthe ultrasonic waves entering the workpieces possess a directionalcharacteristic which is dependent on the phase triggering and whose maindirection of propagation can be swung between 0° and 90° in relation tothe normals to the workpiece.
 46. Use according to claim 44, wherein thecourse of generating ultrasonic waves inside the workpiece, the coilarrangement of the individual ultrasonic transducers are triggered insuch a manner that two directly adjacent magnetic flux guide pieceswhich are oriented in opposite direction are each passed through by amagnetic flux.
 47. An arrangement for coupling-media-free generatingand/or for receiving ultrasonic waves in the form of linearly polarizedtransverse wave in a, respectively from a workpiece, wherein at leasttwo electromagnetic ultrasonic transducers according to claim 20 aredisposed in parallel at a distance to each other in such a manner thatthe longitudinal directions of the respective magnetic flux guide piecesof the individual ultrasonic transducers are aligned in parallel.
 48. Anarrangement for coupling-media-free generating and/or for receivingultrasonic waves in the form of linearly polarized transverse wave in a,respectively from a workpiece, wherein at least two electromagneticultrasonic transducers according to claim 21 are disposed in parallel ata distance to each other in such a manner that the longitudinaldirections of the respective magnetic flux guide pieces of theindividual ultrasonic transducers are aligned in parallel.